A force acting on just part of an extended object (either a solid or a volume of a liquid) can cause all of it to move. That motion is due to the transmission of the force through the object by its material. This paper discusses how the force is distributed to all of the object by a gradient of stress or pressure in it, which creates the local force that directly propels each part of the object. Those gradients resemble the ones created in objects by their weights. An example of the latter is the compressive stress in a column of a building increasing steadily toward its lower end. That gradient occurs because each horizontal section through the column supports all of the weight above it, including the load force pushing down on the column's upper end. The gradient resembles the pressure in a container of liquid increasing with depth in it. Likewise, the weight of a vertically hanging cable causes its tension and tensile stress to increase toward its upper end.

Topics
Compressive stress, Tensile stress
1.
Stress, strain, Young's modulus, and Poisson's ratio are discussed in R. P. Feynman, R. B. Leighton, and M. Sands, The Feynman Lectures on Physics (Addison‐Wesley, Reading, MA), Vol. II, pp. 38‐1–38‐2.
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© 2010 American Association of Physics Teachers.
2010
American Association of Physics Teachers
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